The present invention relates to an optical pickup device to be used for writing and reading data to and from an optical recording medium such as a CD (compact disc), a DVD (digital versatile disc), a Blu-ray disc irradiated with a blue color semiconductor laser, or an HD-DVD, as well as an optical disc drive with the optical pickup device installed therein.
In an optical pickup device to be used for writing and reading data to and from an optical recording medium such as a CD, DVD, a Blu-ray disc irradiated with a blue color semiconductor laser, or an HD-DVD, as well as an optical disc drive incorporating such an optical pickup device, light emitted from a light emitting element such as a laser diode is guided to an objective lens through optical parts, including various lenses, prism, mirror and the like and is focused on an optical recording medium, while light returning from the optical recording medium, (e.g., a component reflected by the optical recording medium, which is of the light emitted from the light emitting element), is received by a photodiode through the objective lens and optical parts including various lenses and mirror and is thereby converted into an electrical signal according to the intensity of the light received by the photodiode.
In assembling the optical pickup device, it is necessary that an optical module (light emitting portion) configured a light emitting element such as a laser diode on a sub-mount mounted on a stem and by integrating the sub-mount with a holder formed of metal or resin, or an optical module (detecting portion) configured by integrating a photodiode for receiving an optical signal with holder should be adjusted to an optimum position optically with respect to a case (also designated an optical pickup case) of the optical pickup device, and should be fixed to the case with use of an ultraviolet curing type adhesive. In this regard, it is required that the mounting position of the optical module on the optical pickup device (e.g., the above-described case) be adjusted in a three-dimensional manner in order to absorb (compensate) the “amount of displacement” of the position where any other part than the optical module is mounted on the optical pickup or the tolerance of the said part itself. In order for ultraviolet light to reach a sufficient depth in the direction of the depth of an adhesive portion (an ultraviolet curing type adhesive) which is used for fixing the optical module and the optical pickup case (optical pickup device) to each other, the optical pickup case and the optical module are bonded with a gap of about 200 to 700 μm therebetween. Further, a flexible printed circuit board for the supply of an electric signal is connected to the optical pickup case and the optical module bonded to the optical pickup case through a wide gap so that stress of the flexible printed circuit board is imposed directly on the bonded portion between the optical pickup case and the optical module. Consequently, the bonded portion of the optical module is apt to be dislocated with respect to the optical pickup case.
On the other hand, in the optical pickup device, the highest positional stability is required of the optical module portion. To meet this requirement, an excellent adhesive able to ensure a positional stability of the optical module relative to the optical pickup case generally tends to have high elastic modulus and hardness. However, such an adhesive is disadvantageous in that the adhesive strength thereof just after the bonding is low and that the adhesive strength is apt to be deteriorated in such an external environment as contains humidity. Therefore, in fixing the optical module to the optical pickup case with use of an adhesive, priority is given to the adhesive strength and there is used an adhesive of relatively high elastic modulus and hardness.
In recent years, since optical disc drives are configured to cope with high-speed recording for an optical disc (optical recording medium), an output amount of laser light in an optical pickup device is increased. Consequently, the amount of heat generated from the optical pickup device also increases. Thus, it is now essential to conduct studies to prevent an increase in temperature of the bonded portion between the optical pickup case and the optical module. To solve the above problem, various improvements have been made for the optical pickup case and the optical module with respect to their materials and structures. Above all, as measures which are highly necessary, there has been proposed a method wherein a resin superior in heat conductivity typified by silicone resin is used as a heat dissipating material and is filled into the adhesive-free spatial portion sandwiched in between the optical module and the optical pickup case and also into a space sandwiched in between the optical module and a cover attached to the optical pickup.
In connection with the optical pickup device, Japanese Patent Laid-Open Publication No. Hei 5 (1993)-210851 (hereinafter referred to as Patent Document 1) discloses a structure for fixing by bonding an optical part disposed in an optical path to a holding member, wherein a portion of an adhesive layer, which is exposed to the exterior, is covered with a coating material to prevent a bad influence from exerting on the adhesive layer from the exterior. Japanese Patent Laid-Open Publication No. 2002-342947 (corresponding to U.S. Pat. No. 6,741,407 B2) (Patent Document 2 hereinafter) discloses that a plate (corresponding to an optical module) which carries a photodiode, etc. thereon is fixed temporarily to a housing (corresponding to an optical pickup case) through a small spacing with use of an ultraviolet curing type adhesive, then an appropriate amount of a heat curing type adhesive is poured between cured ultraviolet curing type adhesive portions and is cured by heating, whereby the drawback of the temporary fixing using the ultraviolet curing type adhesive which is highly elastic with a change in temperature is remedied and the mass productivity of the optical pickup is improved. Japanese Patent Laid-Open Publication No. 2004-10758 (Patent Document 3 hereinafter) teaches a structure for mounting an optical element (e.g., a circuit board carrying a light receiving element thereon) for which a high positional accuracy is required to an optical head assembly (e.g., a frame thereof, corresponding to an optical pickup case), wherein the optical element is fixed to the optical head assembly using a highly thixotropic, filler-rich adhesive containing a large amount of filler and, for the purpose of reinforcing a low adhesive strength which is a drawback of the filler-rich adhesive, a filler-lean adhesive of a low viscosity is provided near a bonding interface between the optical element and the optical head assembly. According to this structure disclosed in the Patent Document 3, even if the optical element is fixed by bonding to the frame of the optical head assembly through a gap, there occurs neither peeling nor a lowering of strength of the bonded portion caused by shrinkage stress at the time of curing of the filler-rich adhesive and a high positional accuracy of the optical element relative to the optical head assembly is ensured.